The present invention relates to a fluidtight seal which is capable of being heat cured between metal objects and a body of vitroceramic material and more particularly to such a method for producing gas lasers, in particular laser gyrometers.
Laser gyrometers are ring lasers whose structure is defined by a block of a vitroceramic material having a very low coefficient of thermal expansion, for example on the market under the name "Zerodur".
The three or four mirrors forming the ring resonant cavity and the metal electrodes (a cathode, two anodes), are fixed on the machined block of vitroceramic material.
The mirrors, whose substrate is also of a vitroceramic material, are usually fixed to the block by molecular adhesion which is a direct connection between two optical, perfectly polished, planar and clean surfaces put into intimate contact with each other.
The electrodes are used for establishing a continuous electric discharge in the gas composed of a mixture of helium and neon, which acts as an amplifier medium for creating the laser effect (emission of neon at 0.633 .mu.m).
The cathode is usually a member of aluminum having a suitable emitting surface and the anodes are for example formed by tungsten rods, although other metallic materials may be used for either type of electrode.
The seals between the metal electrodes and the vitroceramic block of laser gyrometers must satisfy various very severe specifications, mainly for ensuring the durability of the laser gyrometers under precise conditions of use:
fluidtightness with respect to vacuum and helium at 10.sup.-11 atm.cm.sup.3.s.sup.-1 under the normal conditions of temperature and pressure;
absence of pollution of the block when sealing;
limited sealing temperature so as to avoid impairing the properties of the vitroceramic material, thereby ensuring the integrity of the machined block (for example a temperature of &lt;700.degree. C.);
possibility of subsequently reheating the seal to 200.degree. C. at least during a laser conditioning stage (thermal degassing under a vacuum for ensuring the durability of the laser);
lifetime of depending on the applications, more than ten years storage or more than ten thousand hours of operation;
performance in thermal environments of typically between -50.degree. C. and +100.degree. C.;
performance under mechanical and vibratory conditions.
The vitroceramic-metal seals for the electrodes must satisfy various requirements of operation and overall size of the electrodes and economical conditions of manufacture.
Many embodiments are possible. The metal-vitroceramic seal may be effected directly on the electrode or on an auxiliary metal member acting as a case or support for the electrode proper. In this case, such auxiliary member is chosen for its material and geometry to facilitate sealing in accordance with the chosen technique.
The fixing of the metal electrodes has led to the development of techniques for achieving sealing between the metal and vitroceramic.
Most of the methods for achieving a fluidtight seal for gas lasers are based on the use of a seal joint or joining member formed of gold and indium.
U.S. Pat. No. 3,277,281 discloses a method for sealing mirrors or electrodes to a stable laser body of a material having a low coefficient of expansion.
The surfaces to be sealed receive a coating of gold before being put into intimate contact with each other. The seal is produced when the material of a ring of indium, placed outside and heated to its melting temperature, progressively migrates by capillary action between the two surfaces and forms an alloy of Au-In which constitutes a thin sealing joint (.ltoreq.5 .mu.m).
This type of method does not always satisfy the aforementioned fluidtight conditions.
French Pat. No. 2 473 035 discloses for an application to laser gyrometers a method for achieving a seal between a block of vitroceramic and metal electrodes which comprises:
a deposit of gold on the metal member, then the application of a relatively thick blank of indium heated to 175.degree. C. under a vacuum so as to form an alloy Au-In at the interface;
a cleaning of this treated metal surface and of the polished useful surface of the vitroceramic block, this cleaning being carried out under a vacuum by an ionic bombardment or exposure to an ultraviolet radiation;
pressing between the two surfaces which are thus cleaned and devoid of any impurities so as to achieve a seal.
Further, French Pat. No. 2 502 722 discloses another method for obtaining a seal between a block of vitroceramic and metal electrodes for producing laser gyrometers.
This method comprises the following operations:
deposit of a gold coating on the elements of vitroceramic and metal, which are carefully machined;
application of a toric joining member of pure indium between the two surfaces, and then compression in the cold state so as to form a sealing joint having a thickness of about 0.1 mm, this joint being fluidtight at this stage (5.10.sup.-11 atm.cm.sup.3.s.sup.-1);
heating to 140.degree. C. for 12 hours of the assembly thus produced so as to promote diffusion of the indium into the gold.
These various methods and generally the methods involving the formation of a sealing joint which is fusible at low temperature, for example based on indium and indium alloys, have at least two drawbacks.
They require the deposit of a gold coating on the bodies to be sealed, or at least on one thereof, and do not permit a reheating of the sealed assembly to a sufficient temperature, whereas it is desirable to be able to heat to at least 200.degree. C. under vacuum the laser or the gyrometer laser when it is conditioned, so as to remove impurities by degasing and to ensure over a long period the cleanliness of the gas He-Ne used as the amplifying medium. Otherwise, in order to expect to reach the required duration of life, it is necessary to employ traps or "getters" of impurities, the use of which presents drawbacks.
There is moreover another method for achieving a fluidtight seal between glass or glass and metal achieved at a higher temperature and known by the term thermo-compression. This method is disclosed, for example, in the U.S. Pat. No. 2,876,596.
An aluminum joining member is brought to a temperature close to its melting point (500.degree.-600.degree. C.) which temperature is necessarily lower than the softening point of the bodies to be sealed. A pressure of approximately 10 MPa is then exerted between the parts to be sealed so as to produce an intense plastic deformation of the aluminum while breaking the film of oxide (Al.sub.2 O.sub.3) present on the surface of the initial joint. A sealing joint of aluminum having a thickness on the order of 0.1 mm is thus formed.
French Pat. No. 2 193 794 also discloses a method for producing a fluidtight seal with the use of a jointing member of aluminum or lead or an alloy of one of these metals.
The field of application of this patent concerns assemblies between an object of metal and an object of ceramic or glass, and in particular for the production of electronic tubes under a vacuum (image reproducing tubes, filming tubes, X-ray image amplifiers).
In order to achieve fluidtightness over a long period, this patent stresses the necessity of a rapid and intense plastic deformation of the joining member, preferably within less than 2 seconds, so as to avoid the presence of an intermediate layer of oxide in the assembly formed.